The fourth beta of iOS 9.3 fixes a bug that bricked iPhone or iPad models powered with 64-bit processor and running iOS 8 or above. Apple seeded the beta recently to developers and public beta testers.
The issue known as 1970 bug was discovered earlier in the month. The problem occurs when the device's date is manually set to 1 January 1970 or earlier, and results in a continuous reboot. The boot loop is said to be due to an "integer underflow" that causes the iPhone to reset the date to the maximum value. With iOS 9.3 beta 4 running, Apple has made it impossible to set the date of iPhone or iPad beyond 31 December 2000, 7pm ET, which in turn eliminates the 1970 bug.
Apple had earlier acknowledged the issue and said, "An upcoming software update will prevent this issue from affecting iOS devices," without providing any details as to when the update will go live.
The final version of iOS 9.3 will become available in spring and it is highly likely we will see a rollout following Apple's media event on 15 March, when the company is expected to unveil the iPhone 5se, iPad Air 3 and new Apple Watch bands.
Folks at Macrumor forum were the first ones to notice that the iOS 9.3 beta 4 resolves the boot loop issue. A forum member observed, "This update fixed the 1970 date bug. Had two retail units stuck in boot loops do to some pricks setting the date to 1970 and restoring in DFU mode did not help. But restoring to this BETA update made both devices go back to normal."